In order to achieve a power increase with steam power plants in the prior art, the blading of steam turbines is often exchanged for increasing the inner efficiency. Also, a power increase is often achieved by reducing safety margins, i.e. as a rule by increasing pressure flow and mass flow. In a further method which is known in the prior art for retrofitting a steam power plant, a secondary turbine on a secondary drive shaft is connected between the steam heater and the main turbine. In this case, as a rule the main drive shaft is mechanically intercoupled with the secondary drive shaft for driving an electric generator.